//! Helper routines for higher-ranked things. See the `doc` module at
//! the end of the file for details.

use super::combine::CombineFields;
use super::{HigherRankedType, InferCtxt};
use crate::infer::CombinedSnapshot;
use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
use rustc_middle::ty::{self, Binder, TypeFoldable};

impl<'a, 'tcx> CombineFields<'a, 'tcx> {
    /// Checks whether `for<..> sub <: for<..> sup` holds.
    ///
    /// For this to hold, **all** instantiations of the super type
    /// have to be a super type of **at least one** instantiation of
    /// the subtype.
    ///
    /// This is implemented by first entering a new universe.
    /// We then replace all bound variables in `sup` with placeholders,
    /// and all bound variables in `sup` with inference vars.
    /// We can then just relate the two resulting types as normal.
    ///
    /// Note: this is a subtle algorithm. For a full explanation, please see
    /// the [rustc dev guide][rd]
    ///
    /// [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html
    #[instrument(skip(self), level = "debug")]
    pub fn higher_ranked_sub<T>(
        &mut self,
        sub: Binder<'tcx, T>,
        sup: Binder<'tcx, T>,
        sub_is_expected: bool,
    ) -> RelateResult<'tcx, ()>
    where
        T: Relate<'tcx>,
    {
        let span = self.trace.cause.span;

        self.infcx.commit_if_ok(|_| {
            // First, we instantiate each bound region in the supertype with a
            // fresh placeholder region. Note that this automatically creates
            // a new universe if needed.
            let sup_prime = self.infcx.replace_bound_vars_with_placeholders(sup);

            // Next, we instantiate each bound region in the subtype
            // with a fresh region variable. These region variables --
            // but no other pre-existing region variables -- can name
            // the placeholders.
            let sub_prime =
                self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, sub);

            debug!("a_prime={:?}", sub_prime);
            debug!("b_prime={:?}", sup_prime);

            // Compare types now that bound regions have been replaced.
            let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?;

            debug!("higher_ranked_sub: OK result={result:?}");
            // NOTE: returning the result here would be dangerous as it contains
            // placeholders which **must not** be named afterwards.
            Ok(())
        })
    }
}

impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
    /// Replaces all bound variables (lifetimes, types, and constants) bound by
    /// `binder` with placeholder variables in a new universe. This means that the
    /// new placeholders can only be named by inference variables created after
    /// this method has been called.
    ///
    /// This is the first step of checking subtyping when higher-ranked things are involved.
    /// For more details visit the relevant sections of the [rustc dev guide].
    ///
    /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
    #[instrument(level = "debug", skip(self))]
    pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
    where
        T: TypeFoldable<'tcx> + Copy,
    {
        if let Some(inner) = binder.no_bound_vars() {
            return inner;
        }

        let next_universe = self.create_next_universe();

        let fld_r = |br: ty::BoundRegion| {
            self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
                universe: next_universe,
                name: br.kind,
            }))
        };

        let fld_t = |bound_ty: ty::BoundTy| {
            self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
                universe: next_universe,
                name: bound_ty.var,
            }))
        };

        let fld_c = |bound_var: ty::BoundVar, ty| {
            self.tcx.mk_const(ty::ConstS {
                kind: ty::ConstKind::Placeholder(ty::PlaceholderConst {
                    universe: next_universe,
                    name: ty::BoundConst { var: bound_var, ty },
                }),
                ty,
            })
        };

        let result = self.tcx.replace_bound_vars_uncached(binder, fld_r, fld_t, fld_c);
        debug!(?next_universe, ?result);
        result
    }

    /// See [RegionConstraintCollector::leak_check][1].
    ///
    /// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check
    pub fn leak_check(
        &self,
        overly_polymorphic: bool,
        snapshot: &CombinedSnapshot<'_, 'tcx>,
    ) -> RelateResult<'tcx, ()> {
        // If the user gave `-Zno-leak-check`, or we have been
        // configured to skip the leak check, then skip the leak check
        // completely. The leak check is deprecated. Any legitimate
        // subtyping errors that it would have caught will now be
        // caught later on, during region checking. However, we
        // continue to use it for a transition period.
        if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() {
            return Ok(());
        }

        self.inner.borrow_mut().unwrap_region_constraints().leak_check(
            self.tcx,
            overly_polymorphic,
            self.universe(),
            snapshot,
        )
    }
}
